Jars and similar containers which are packed with a food product under vacuum (having less than atmospheric pressure in the head space above the food product) are more difficult to open than those which are not packed under vacuum. Atmospheric pressure above the closure exceeds the pressure in the head space beneath the closure, so that a net pressure differential force acts downwardly on the closure to hold it on the container. Since this pressure force is proportional to the area of the mouth of the container, it increases as the square of the radius of the container increases. Moreover, this pressure force rapidly increases as the size of the mouth of the container increases. Because the pressure force acts in conjunction with the frictional force of the closure threads, lugs, snaps, or other securing means to hold the closure on the container, it is much more difficult to remove a closure on a vacuum packed product. If the closure is unitary with an integral top panel screw thread, the mechanical friction between it and the container threads and the vacuum force must be overcome simultaneously. This occurs with these types of closures as they are rotated off the containers.
So-called composite closures, in which a separate insert disk or lid is rotatable within a threaded skirt or shell, facilitate opening vacuum packed containers because the shell can be rotated on the container to overcome the starting or mechanical friction without at the same time rotating the disk on the top or breaking the vacuum. Once the starting or mechanical friction is overcome, the threads gradually lift the disk and break the vacuum.
However, the problem is more difficult with closures of the so-called "press-on, pry-off" type, which are not removed by rotation. Such closures have a protrusion inside the shell which snaps beneath a snap rib on the container finish to secure the closure. Since no threads or lugs provide a mechanical advantage to lift the insert disk, the closure must be removed by prying it upwardly, as with a thumb positioned on its lower edge or an outwardly projecting tab, so as to force the shell protrusion over the snap rib. Both the pressure differential force and the tension of the snap must also be simultaneously overcome by the upward lifting force. Indeed, the required lifting force is so great that press-on, pry-off closures are impractical for some vacuum packed product containers, especially if the container's mouth diameters are greater than about 72 mm., unless an outwardly extending thumb tab is provided to give the needed leverage.
The force required to press open a press-on, pry-off closure is greater still if a tamper evidencing band is present. Such bands are designed to break or tear away before the closure can be opened or the contents interfered with, and are widely used to provide a visible indication if the closure has been partially or fully opened. Breaking the band adds another resistance which must be overcome, and thus further increases the pry-open force required.
In addition to the above-mentioned problems associated with removing closures from vacuum-packed containers, there is the ever increasing problem of a "dirty finish" on the container outer necks and snap ribs resulting from faster container filling and capping speeds. In other words, as the container filling and capping speeds increase, the more likely it is that the contents, such as food products, with which the containers are to be filled, will spill or splash onto the outer necks and snap ribs of the containers.
This "dirty finish" is also a common occurrence with those containers that are subject to retorting, i.e., following container filling and capping, food or vacuum-packed containers are cooked to temperatures on the order of about 250.degree. F. to sterilize the food contents filled therein. During the retorting process, if there is less than, for example, a 6% head space left in the containers due to overfill or if there is too much pressure within or too little pressure outside of the food or vacuum-packed containers, the hydraulic forces within the containers will cause the vacuum seals to break and the inner food contents to seep between the container rims and the closures and then onto the containers, outer necks and snap ribs.
In either situation, the "dirty finish" on the container necks and snap ribs presents a sanitation problem if the spilled, splashed or seeped materials are permitted to remain and dry thereon. For example, if certain food residues are left on the outer necks and ribs of the containers to dry, mold growth, entrapped moisture, infestation of fruit flies or the like can result. Thus, a "clean finish" on the outer necks and snap ribs of the containers is required in order to meet and pass the USDA's FSIS requirements.
Still further, due to environmental concerns, there is an ever increasing demand to recycle plastic and glass containers such as described above. There is also an ever increasing demand to provide containers such as those described above with tamper evidencing indicators to advise consumers in advance as to whether the containers have or have not been tampered with. This is generally accomplished by providing the closures for containers with tamper evidencing bands which break free from the closures upon removal of the closures from the containers to open the containers. Unfortunately, a drawback associated with tamper indicating bands available heretofore is that, once they have been broken free from the closures, they remain secured around the necks of the containers. This drawback presents a recycling problem concerning the used containers, and in particular used glass containers, since the tamper evidencing bands which are secured to the container necks must be first cut free and removed therefrom before the used containers can be recycled, reused if not recycled.
Microwave ovens have become widespread in recent years, and have provided a way to rapidly and conveniently cook many types of foods. Unfortunately, unfrozen, shelf-stable microwave packages or containers available heretofore have been inconvenient if not unsatisfactory. A typical unfrozen, shelf-stable microwave package currently available is a plastic laminate tub having a plastic snap-on, pry-off dust cover or lid. To prevent contamination and to improve shelf stability, the opening of the plastic tub is sealed with a metal lid. Prior to microwaving the food in the packages, however, the plastic dust covers or lids must be pryed-off and the metal lids carefully removed. If the metal lids are not removed prior to microwaving the packages, the packages can split or rupture during the microwaving process permitting the food to leak out therefrom. once the metal lids have been removed, the plastic dust covers or lids are snapped back on and the packages are then ready for cooking in microwaves.
To vent pressure or steam that may build internally within such packages during the microwave cooking process, the plastic dust covers of such packages are generally formed with a few through-holes. When these types of microwave packages are cooked in residential wattage microwaves, i.e., a maximum of about 750 watts, the plastic dust covers normally will not blow off and will guard against food splatter. However, if they are used in industrial wattage microwaves, i.e., near about 1,000 watts or greater, the few holes in the dust covers are sometimes to be insufficient, and consequently, the plastic dust covers or lids will either blow off or be ineffective in preventing food splatter, or both.
As an alternative to the plastic tub microwavable packages, glass containers having twist or screw-on type covers have been proposed. A major disadvantage with these types of packages, however, is that if the covers are not loosened or entirely removed prior to cooking, they can become potentially even more explosive due to the increase in internal pressure and steam resulting from the cooking process. For instance, a large diameter twist cap will hold approximately 10-15 lbs. of internal pressure before it vents or the package bursts.
As part of the manufacturing and packaging operations, most vacuum-packed food packages, including plastic tub microwavable food packages, are "retorted" to precook and sterilize the food product contained therein. Typically they are almost or completely filled, e.g., on the order of about 94% to 100% filled with food product. (This degree of filling corresponds to a "head space" of about 6%-0% of the internal space above the food product in the container.) The retorting process basically requires that the vacuum-packed food containers be cooked at high temperatures for prolonged periods of time. The retorting vessel is usually under pressure, to keep the sealed containers from bursting. To ensure that the entire contents is properly precooked and sterilized, the outer portion of the contents is somewhat overcooked during the retort process. Unfortunately, the retort process, as presently performed, requires significant amounts of energy and labor costs. More importantly, however, the retort process as conducted up to now substantially diminishes the quality of these vacuum-packaged food products the standpoint of flavor, color and texture. Because of this shortcoming associated with the retort process, frozen foods are generally preferred over vacuum-packed, retorted, shelf-stable food products.
Up to now those skilled in the packaging arts have been unsuccessful in achieving an effective, long-term hermetic seal that is also resealable without compression at the sealant interface. Seals which are generally only achieved by tightly torquing helical threads or lugs, or seal compression principally achieved by an internal package vacuum or both, do not typically maintain an effective, long-term hermetic seal. The hand press-on, pry-off and resealable closures, which are less common than threaded or crimped-on package closures available heretofore, require an internal vacuum to effect and maintain a long term hermetic seal. However, as a vacuum is inappropriate with some food products, for instance, or is unachievable in most plastic containers, etc., the press-on closure seals are not considered a viable solution to forming and maintaining an effective long-term hermetic seal. As an alternative, seals formed with an adhesive sealant (potentially with a residual tack for a more effective reseal) have not been suitable because they generally cannot be easily pried-off or be adequately resealed.
Consequently, there is a demand in the industry for press-on, pry-off closures which can be more easily removed from containers by consumers. Further, there is a demand for packages which can be retorted more quickly, and in which the food product will have better flavor, color, and texture after retorting. In addition, there is a demand in the industry for capped closures which have "clean finishes" on their outer necks and snap ridges following the filling and capping procedures as well as the retorting process. Still further, there is a demand in the industry for closures which facilitate the removal of the tamper indicating bands from used containers to simplify the use, reuse and recycling process of the used containers.